Silica-alumina catalysts

ABSTRACT

Porous silica-alumina products useful in catalysis are prepared by adding excess alkali to a body of water glass, the excess alkali is responsible for rendering the product extrudable and for increasing the pore diameter.

According to U.S. Pat. No. 2,933,456 a silica-alumina complex useful incatalysis is prepared in the following manner:

1. SILICA IN HYDROUS FORM IS PRECIPITATED BY ADDING A MINERAL ACID(SULFURIC OR HYDROCHLORIC) TO A BODY OF WATER GLASS, Na₂ O (SiO₂)₃.2, pH8 to 10.5;

2. ADDING TO THE RESULTANT SLURRY (SILICIC ACID GEL) AN ACIDIC AQUEOUSSOLUTION OF AN ALUMINUM SALT SUCH AS ALUMINUM SULFATE OR ALUMINUMCHLORIDE HAVING ALUMINUM IN THE CATION PORTION, LOWERING THE PH to 2 or3, to precipitate alumina in hydrous form on to the silica;

3. WHEREAFTER AN AQUEOUS SOLUTION OF AN ALUMINUM SALT IN WHICH ALUMINUMEXISTS IN THE ANION (SODIUM ALUMINATE OR POTASSIUM ALUMINATE) IS ADDEDTO THE SILICA-ALUMINA SLURRY TO PARTIALLY NEUTRALIZE THE ACIDITYPRODUCED BY THE FIRST ALUMINUM SALT, PREFERABLY AT A PH of about 5,which impregnates the silica gel with alumina precipitated from bothaluminum salts;

4. THE RESULTANT SLURRY CONTAINING SILICA IMPREGNATED WITH ALUMINA ISFILTERED TO REMOVE FREE MOISTURE AND THE FILTER CAKE IS SPRAY DRIED TOOBTAIN THE CATALYST CARRIER IN POWDER FORM WHICH MAY BE RESLURRIED ANDREFILTERED TO REMOVE SOLUBLE SALTS.

In summary: in the known process of U.S. Pat. No. 2,933,456 silicaalumina catalyst is obtained by precipitating silica in hydrous formfrom aqueous sodium silicate by means of a mineral acid and subsequentlyimpregnating the silica gel with alumina precipitated in hydrous form bysuccessive additions, first, of a water soluble aluminum salt containingaluminum in the cation only and, second, a water soluble aluminum saltin which aluminum exists in the anion. Most likely there isco-precipitation of silica and alumina during aluminum addition. Aftersuccessive filtering and spray drying steps the final purified productis a powder where the silica content is 55 to 95% by weight (dry basis)and alumina the balance. The amount of aluminum sulfate (alum)determines whether the product is high or low in alumina.

The particles are microspherical, inherently porous and susceptible toimpregnation with metallic catalyst precursors such as compounds ofmolybdenum, cobalt, nickel, tungsten, or mixtures thereof. Somereactions to be catalyzed take place on the surface of the particles andsome reactions are promoted by diffusion into the catalyst supportparticles. In the first case, the pores are permissably (and preferably)small so as to confine the catalyst to the surface of the particles. Inthe second case the pores should be larger to assure penetration of thecatalyst into the carrier. These are general cases; there areexceptions.

The commercial product under U.S. Pat. No. 2,933,456 is represented bymicrospherical particles obtained by spray drying. The microspheres maybe of 50 micron diameter, employed in a fluidized bed where theparticles are subjected to a great deal of contact one against another,both impact and sliding, resulting in fine subdivisions (as small as say10 micron diameter) which can clog the catalytic reactor. The catalystbed itself shifts, resulting in a grinding action which is alsoproductive of fractured particles.

In some catalytic processes, particles of larger size and ofconsiderable strength are required. Such sizes could be obtained byextrusion, resulting in pellets composed of numerous microspheres.However, extrusion has not been possible on an economic basisprincipally because the material undergoes wall slippage, that is, thebulk to be extrudable must be wet and being wet it slips so much insidethe chamber behind the extrusion nozzle, where the force is applied,that only slow, uneconomic rates can be achieved, and even then thepellets are not strong.

One object of the present invention is to enable a catalyst support ofthe kind involved to be extruded, preferably having a crush strength ofat least eight pounds in the 1/8 inch - 5/64 inch diameter size, andanother object is to be able to produce the catalyst support with agreater proportion of pores in a larger diameter than heretofore.

Under the present invention, extrudability of microporous silica-aluminacatalyst carriers of exceptional strength can be achieved in a practicalproduction sense by following the process of the patent except that thestarting body of water glass, Na₂ O (SiO₂)₃.2, is infused with excessalkali. Excess alkali is preferably in the form of caustic (NaOH) orsodium carbonate, Na₂ CO₃. By employing excess alkali, the resultantparticles are opaque and more soft compared to the clear, hard,glass-like particles obtained under U.S. Pat. No. 2,933,456 where excessalkali does not exist. Indeed, the clear, hard, glass-like particles ofU.S. Pat. No. 2,933,456 cannot be extruded to produce strong, useableextrudates.

Excess alkali means more than is present in water glass. In water glassthe amount of alkali is precisely as reported in U.S. Pat. No.2,933,456, namely, in the weight proportion of Na₂ O (SiO₂)₃.27, that is1:3.27. In orthosilicate, for comparison the mole ratio is 2Na₂ O:SiO₂and in meta silicate the mole ratio of alkali to silica is 1:1.

Under Example 1 below excess alkali is obtained by adding NaOH to"plant" sodium silicate which is water glass.

EXAMPLE 1

6,720 mls of aqueous sodium silicate containing excess alkali wereprepared by digesting 3,160 mls plant sodium silicate in 3,560 mls of50% NaOH at 170° F. for 15 minutes, and added to a tank containing 10.8gallons water (90° F.). Then 4,920 mls of plant sodium silicate, waterglass (9.1% Na₂ O, 28.8% SiO₂), were added and the solution was heatedto 120° F. To the above solution, 10,930 mls of 35% H₂ SO₄ were addedwith agitation over a period of 50 minutes to pH 8.3. To the aboveprecipitated silica gel, 6,420 mls of plant concentrated alum (25%aqueous solution) were added over a period of 17 minutes. The mixturethen was adjusted to pH 5.5 with 11,292 mls of diluted sodium aluminate(SA) solution (1,830 mls of plant SA diluted in 9,462 mls H₂ O). Thediluted SA solution was added over a period of 20 minutes. The finalbatch slurry temperature was 108° F.

The batch slurry was filtered in buchner funnels. The filter cake wasreslurried in 10 pounds of diethylene glycol and was spray dried. Thedried product was washed with water and with a dilute NH₄ OH to removethe soluble salts. The purified filter cake was oven dried at 150° F.for 16 hours. The dried product was adjusted with water to 62% FM (freemoisture) using a Simpson muller. The wet material was mulled for 20minutes and was extruded. Extrusion was very good. The extrudates werecalcined at 1100° F. for 3 hours. Data on crush strength and poredistribution are shown in Table I for the 5/64 inch size. The "freemoisture" is calculated as the percent weight loss by heating a sampleof the mulled or extruded material on a Model 6000 OHAUS MoistureDetermination Balance for 20 minutes at a heater setting of 75.

EXAMPLE 2

The same formulation and procedure were used in preparing this catalystas in Example 1 with the exception the amounts of alum and sodiumaluminate were increased slightly to obtain 31% Al₂ O₃ on the catalyst.The material was extruded at 64.5% FM. Extrusion was very good. Data aregiven in Table I.

EXAMPLE 3

To increase the density, the same formulation and procedure were used asin Example 2 with the following exceptions, (a) the sodium silicatecontaining excess alkali was prepared in 16 gallons H₂ O and (b) only0.1 pound of diethylene glycol was used instead of 1.0 pound per poundof catalyst. Extrusion was very good. Data are given in Table I.

EXAMPLE 4

This sample is a repeat of Example 2 with the exception that thematerial was extruded at 63% FM instead of 64.5%. Data are given inTable I.

EXAMPLE 5

A repeated preparation of Example 2 except the purified spray driedproduct was calcined at 1050° F. for 3 hours and prepared in powderedform rather than extruded. Data are given in Table I.

EXAMPLE 6

This example was prepared by the method of Example 1 herein but using noexcess alkali thus conforming to Example 1 of U.S. Pat. No. 2,933,456.The product could not be extruded to give a material of useable crushstrength. This example also illustrates the amount of pores below 100Awhich are generally not affected by extrusion, which is to say thatwhile the product of Example 6 was not extruded the pore volume below100A may be validly compared to the corresponding pore volume of theextrudates set forth in Table I.

                  TABLE I                                                         ______________________________________                                                       Crush    Pore Volume (PV)                                                                         1200- 0-                                   Ex-   Extruded Strength Less Than 1200A°                                                                  100A°                                                                        100A°                         ample Diameter (pounds) Diameter (1)                                                                             (PV)  (PV)                                 ______________________________________                                        1     5/64     9.5                                                            2     5/64     9.1      1.15       0.98  0.17                                 3     5/64     11.2     0.87       0.71  0.16                                 4     5/64     8.5      0.93       0.78  0.15                                 5                       1.46       1.33  0.13                                 6                       0.65       0.30  0.35                                 ______________________________________                                         (1) By nitrogen absorption; cc/gm                                        

Diethylene glycol is only an aid to extrusion. It has no appreciableeffect on strength or pore volume distribution. Each of Examples 2through 5 was of high alumina content (approximately 31% by weight).Example 6 had the same alumina content for comparison.

It will be seen from Table I that extrudates with good crush strengthare realized. Crush strength is tested simply by applying an increasingforce until the pellet fails in compression.

It is believed the advantageous result is because the excess alkaligenerates softer catalyst particles more capable of conforming to theextrusion orifices. However, a completely unexpected result is the shiftin pore volume distribution. Thus, in comparing Example 6, Table I, tothe others, excess alkalinity results in nearly a fifty percent decreaseof pore volume below 100A, that is, the known catalyst of Example 6 hadconsiderably more pore volume in the smaller diameter.

That increased alkalinity (deemed Na₂ O) is responsible for the shift inpore volume is corroborated by sodium carbonate (Na₂ CO₃) accomplishingthe same thing, as shown by the following examples.

EXAMPLE 7

A catalyst was prepared using water glass in accordance with Example 1of U.S. Pat. No. 2,933,456, without excess Na₂ O. This catalyst was lowin alumina, approximately 13% by weight. It was not possible to extrudethis catalyst. This example is included as a pore distributioncomparison catalyst.

EXAMPLE 8

This catalyst was a repeat of Example 7 except sodium carbonate (Na₂CO₃) was added to the body of water glass at the inception in theproportion of 0.32 pounds of Na₂ CO₃ per pound of final catalyst.

EXAMPLE 9

This catalyst was a repeat of Example 8 except the amount of Na₂ CO₃ wasreduced to 0.16 pounds per pound of final catalyst.

                  TABLE II                                                        ______________________________________                                                PV Less Than                                                          Example 1200A°       PV 100A° or Less                           ______________________________________                                        7       0.69                0.47                                              8       1.10                0.19                                              9       0.98                0.27                                              ______________________________________                                    

The catalyst prepared without excess alkali (Example 7) had nearly 68%of the pore volume in the 100A size or less, compared to only 17% forExample 8 and 27.5% for Example 9. Examples 8 and 9 show that as theproportion of excess alkali increases the greater the pore volume in thelarger diameter. This is apparent from the drawing where percent porevolume in the smaller size (100A or less) is plotted against excessalkali; the amount of excess alkali in Example 8 is deemed of unit valueand by that token the amount of excess alkali under Example 9 isone-half unit. In other words, the greater the proportion of alkali inexcess of that required to form water glass, the greater the porediameter.

The preferred example is Example 1 where the amount of sodium hydroxide(calculated as pure NaOH) is 0.55 pounds per pound of dry, finishedcatalyst obtained after extrusion and calcining. On a mole basis theexcess alkali (derived from NaOH) is 2.04 moles Na₂ O per mole of waterglass. The excess alkali is simply that required to render theoven-dried, purified filter cake extrudable (on a commercial scale) inthe presence of free moisture; or viewed another way the excess overthat required to form water glass in an amount sufficient to producepores of an appreciably larger diameter compared to the pore sizedistribution when the body of sodium silicate is in water glassproportion.

Extrusion aids are common; the present process is not necessarily anexception but of course the extrusion aid is eliminated duringcalcination.

We claim:
 1. In a process where porous microspherical silica-aluminaparticles useful in catalysis are prepared by initially adding a mineralacid to a body of aqueous sodium silicate starting material initially toprecipitate silica in hydrous form, followed by addition of an aluminatesalt to precipitate hydrous alumina on the silica, after which theproduct is filtered, purified and dried: the improvement characterizedby said body of aqueous sodium silicate starting material, before acidis added, being treated with Na₂ O so that the alkali, expressed as Na₂O in the starting material, is substantially in excess of that requiredto form water glass and in an amount sufficient to produce pores of anappreciably larger diameter compared to pore size distribution when thebody of sodium silicate is in water glass proportion without excessalkali.
 2. A process according to claim 1 further characterized by thesteps of adding water to the purified, dried particles to obtain anextrudable mixture; and afterwards extruding and calcining the mixtureto afford the desired catalyst support bodies.
 3. A process according toclaim 1 in which excess alkalinity is achieved by addition of sodiumhydroxide or sodium carbonate to water glass.